This invention relates to instruments to measure the profile of large, smooth optical-quality surfaces and more particularly to such instruments utilizing pencil beam interferometry.
The design and fabrication of exotic, unconventional optical components have advanced beyond the capability of traditional metrology techniques to measure and qualify those components. Single-point diamond turning techniques have improved significantly over the past few years so that optical components whose size and shape could not have been imagined just a few years ago are now integral parts of advanced optical systems.
The proliferation of synchrotron radiation sources of high intensity soft x-rays over the past several years, and advances in x-ray laser and short wavelength free electron laser technologies have spurred the development of industries producing far off-axis aspheric optics, such as cylinders and toroids, for extreme grazing incidence applications. For example, optical components employed in the design of synchrotron radiation beamlines are almost without exception used at extreme grazing incidence in order to reflect x-rays with energies in the 1 to 20 kilovolt range. Grazing angles of 10 mrad or less are typical and the surface figures often are plane cylinders, toroids, ellipsoids, and plane surfaces bent into the shape of a toroid or ellipsoid. The tolerances on mirrors such as these are such that slope errors must be measured to better than a arc second in precision and accuracy over a wide range of surface spatial periods.
The surface roughness of mirrors such as those mentioned above is measured in many laboratories with optical profilers which cover the spatial period range typically from 3 microns up to 5 mm. What has been lacking up to now is an instrument capable of measuring surface profile or slope over the mid-to low-frequency range, covering spatial periods from 1 mm up to the full mirror length of 1 meter, in a non-contact manner. The fabrication community and the end user community have been hampered by the lack of suitable metrology instrumentation for rapid assessment of the surface figure of large aspheric optical components.
Conventional interferometric testing of large or aspheric optics, using commercially-available systems, is often not possible or requires an enormous expenditure of resources to develop a facility to fabricate null lens components, e.g., a computer-generated hologram, that in itself requires a complete qualification program. These auxiliary components are usually specific to a single optical element under test and cannot be modified to accommodate changes in the final component design without the need to generate a completely new auxiliary component. It is thus difficult to take advantage of the ability of the precision machining process to rapidly accommodate changes in the design parameters of the optics.
Other scanning instruments based on different optical principles already exist, some as commercial products, for example, as described in Eastman, J. M. and Zavislan, J. M., "A New Optical Surface Microprofiling Instrument," Proc. SPIE 429 (1983); and Sommargren, G. E., "Optical Heterodyne Profilometry," Appl. Opt. 20, 610 (1981), but each has one or more specific drawbacks limiting substantially its possible use for lens systems described above.
A number of non-contact optical scanning profilers are available and the majority can be categorized into two basic types: (1) those based on probe beams that are focussed onto the test surface, such as Eastman et al identified above, and Hartman, J. S., Gordon, R. L., and Lessor, D. L., "Development of Nomarski Microscopy for Quantitative Determination of Surface Topography," Proc. SPIE 192, 223 (1979); Kohno, T., Ozawa, N., Miyamoto, M., Musha, T., "Practical Non-Contact Surface Measuring
Instrument With One Nanometre Resolution," Prec. Eng. 7, 231-232 (1985); Makosch, G. and Solf, B., "Surface Profiling By Electro-Optical Phase Measurements," Proc. SPIE 316, 42 (1981).; and Whitefield, R. J., "Noncontact Optical Profilometer," Appl. Opt. 14 , 2480-2485 (1975); and (2) optical lever devices that sense the angular change of a probe beam reflected from a test surface, such as Price, R. H., "X-ray Microscopy Using Grazing Incidence Relection Optics," Low Energy X-ray Diagnostics, D. T. Attwood and B. L. Henke, eds., AIP Conference Proc. 75, 189 (1981); Ennos, A. E. and Virdee, M. S., "High Accuracy Profile Measurement of Quasi-Conical Mirror Surfaces by Laser Autocollimation," Prec. Eng. 4, 5-9 (1982); and DeCew, Jr., A. E. and Wagner, R. W., "An Optical Lever for the metrology of Grazing Incidence Optics," Proc. SPIE 645, 127-132 (1986).
Instruments based on focussed probe beams suffer from a limited depth of field, which place severe restrictions on the type of surface that can be measured. Surfaces must be nearly flat, since any slight curvature will cause the position of the surface to exceed the depth of field of the microscope objective. This problem is especially acute on large optical components where even a slight curvature results in a large sag in the center of the optics. Even flat surfaces need to be levelled with great precision so that residual tilt will not cause the surface to exceed the depth of field over long distance.
One method to circumvent the depth-of-field problem is to use an auxiliary metrology system to measure the amount of vertical travel required to refocus the optical head, but this method adds an unwarranted degree of complexity to the problem. To our knowledge this has not been implemented in any of the focussed-beam scanning devices.
The class of profilers based on optical lever principles do not suffer from depth of field problems, since the probe beams are generally unfocussed laser beams, but they share with the microscope-based systems a practical limitation related to the ability of the operator to align the scan axis of the profiler with the symmetry axis of the components in the form of cylinders with minor radii on the order of tens of centimeters or less.
In recent years this problem has been addressed by newly developed optical systems, many of them employing highly coherent, or laser, beam technology, such as for example the so-called pencil beam interferometer. A typical pencil beam interferometer employs a single laser beam split into two colinear beams which are then reflected off of an optical surface. The reflected beams are made to recombine and produce an interference pattern in the focal plane of a Fourier transform lens. Analysis of the interference pattern as the interferometer is traversed across the optical surface yields information on the overall shape of the surface and deviations of the surface from the ideal shape.
A pencil beam interferometer is shown in U.S. Pat. No. 4,498,773 to von Bieren of the type to which this invention relates. The paths which the split beams follow in the patent are not exactly of the same length (i.e., not zero path difference) and in fact the design in the patent is inherently incapable of obtaining the zero path difference with the consequence that a coherent beam, such as a laser beam, is required, and the apparatus is highly sensitive to wind, changes in temperature, and vibration since these conditions will alter the beam lengths differentially. In addition, where alignment of the mirror being analyzed is required, von Bieren does not have any way to utilize the pencil beams to effect this alignment so that other apparatus must be employed for this purpose.
Other U.S. Patents related in subject matter are 2,583,596, 3,661,463, 4,027,976, 4,153,370, 4,170,401, 4,353,650, 4,379,633, 4,385,835, 4,643,576, and 4,647,206.
None of the aforementioned patents is capable of producing the alignment and rapid assessment of the surface figure of large aspheric optical components with the speed and degree of precision required.